CN111542661B - Method and system for adjusting S/Na balance of pulp mill - Google Patents

Method and system for adjusting S/Na balance of pulp mill Download PDF

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CN111542661B
CN111542661B CN201880084435.5A CN201880084435A CN111542661B CN 111542661 B CN111542661 B CN 111542661B CN 201880084435 A CN201880084435 A CN 201880084435A CN 111542661 B CN111542661 B CN 111542661B
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gas stream
pulp mill
aqueous
bioreactor
mill
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CN111542661A (en
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R·哈梅莱伊宁
S·图奥米涅米
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Valmet Technologies Oy
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Valmet Technologies Oy
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Priority claimed from FI20176189A external-priority patent/FI129615B/en
Priority claimed from FI20176188A external-priority patent/FI129614B/en
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    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21CPRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00Regeneration of pulp liquors or effluent waste waters
    • D21C11/06Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
    • D21C11/08Deodorisation ; Elimination of malodorous compounds, e.g. sulfur compounds such as hydrogen sulfide or mercaptans, from gas streams
    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21CPRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00Regeneration of pulp liquors or effluent waste waters
    • D21C11/06Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21CPRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00Regeneration of pulp liquors or effluent waste waters
    • D21C11/0007Recovery of by-products, i.e. compounds other than those necessary for pulping, for multiple uses or not otherwise provided for
    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21CPRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00Regeneration of pulp liquors or effluent waste waters
    • D21C11/0014Combination of various pulping processes with one or several recovery systems (cross-recovery)
    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21CPRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00Regeneration of pulp liquors or effluent waste waters
    • D21C11/0057Oxidation of liquors, e.g. in order to reduce the losses of sulfur compounds, followed by evaporation or combustion if the liquor in question is a black liquor

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Abstract

The present invention relates to a method and a system for adjusting the S/Na balance of a pulp mill, wherein an aqueous pulp mill liquor containing sulphides is transferred to a bioreactor and oxidized by sulphur oxidising microorganisms, thereby producing an aqueous suspension from which elemental sulphur in the form of precipitate can be separated, while the remaining solution can be led to causticisation. Optionally, the aqueous slurry plant liquid may be stripped to obtain an H-containing stream prior to oxidation in the bioreactor 2 S, then subjecting the H-containing gas stream to 2 The gas stream of S is washed with a washing solution to obtain an aqueous spent washing solution containing sulphide, in which case the remaining solution can be used to replenish the washing solution.

Description

Method and system for adjusting S/Na balance of pulp mill
Technical Field
The present invention relates to a method and system for adjusting the S/Na balance of a pulp mill. Aspects of the invention relate to a method and system for separating sulfur from pulp mill liquor. Aspects of the present invention relate to a method and system for bio-oxidizing sulfur compounds in a pulp mill fluid within a pulp mill.
Background
Industrial pulping processes, particularly chemical pulping processes, are used to remove hemicellulose and lignin from wood raw materials to provide cellulosic fibers. Chemical cooking processIn particular kraft cooking, a combination of high temperature and pulping chemicals is used to break the chemical bonds of lignin, a natural biopolymer in wood that binds cellulose fibers together. In the kraft process, wood material is mixed in a digester with an aqueous solution of pulping chemicals and then heated with steam. One example of a sulfate process is the Kraft pulping process (Kraft process), in which the primary pulping chemicals are sodium hydroxide (NaOH) and sodium sulfide (Na 2 S). Chemical cooking processes separate cellulosic fibers from lignin and hemicellulose components and produce spent cooking liquor, referred to as black liquor. This liquid, which contains spent cooking chemicals and byproducts, is then concentrated and typically burned to recycle the cooking chemicals. The recycling of cooking chemicals is often referred to as a pulp mill's liquid cycle or chemical recovery cycle.
As regulations concerning environmental protection become more stringent, modern pulp mills need to recycle chemicals more carefully and seek to reduce the accumulation of sulfur compounds in the environment. A conventional method for treating sulfur-containing side streams formed in pulp mill processes is to dump the side streams as fly ash or recycle the sulfur-containing side streams to other processes for the production of industrial chemicals. One example of sulfur recovery is the combustion of malodorous gases formed as a byproduct of the slurry production process. The combustion of malodorous gases produces flue gas containing sulfur oxides which can be recovered and further used in the manufacture of, for example, sulfuric acid. Sodium bisulphite, sodium dithionite and gypsum are other examples of possible products that may be produced from the sulfur-containing side streams of a pulp mill. However, refining pulp mill flue gas or sulfur-containing side streams into more valuable chemicals requires significant capital investment and separate chemical equipment. Refining may also be problematic from an environmental point of view. In addition, this investment is time consuming and may be difficult to retrofit into processes already existing in conventional pulp mills.
Sulfur is a key chemical in the chemical digestion process of kraft pulp mills and needs to be continuously removed and replenished from the chemical recovery cycle. One particular disadvantage associated with conventional methods for recovering sulfur from pulp mills is the concomitant loss of sodium during chemical digestion, which is typically recovered along with sulfur. This results in the loss of two key elements in the cooking chemicals, which is undesirable for the S/Na balance of the pulp mill. Therefore, in view of the stricter regulations, how to reduce the total sulfur content in the chemical recovery cycle and how to improve the S/Na balance of the pulp mill has been a challenge. Accumulation of sulfur in the chemical recovery cycle is a continuing challenge for efficient operation of the pulp mill. Thus, there is a need for a cost effective and environmentally friendly method and system for controlling the S/Na balance of a pulp mill that is easier to implement on existing processes in conventional pulp mills.
Disclosure of Invention
The problems disclosed above can be solved by providing a method and system that is capable of adjusting the S/Na balance of a mill by separating sulfur compounds from a sulfide-containing mill liquor, such as green liquor or white liquor, and oxidizing the sulfide to elemental sulfur with microorganisms. One advantage is that the total sulfur content in the mill process can be reduced, since the circulation of sulfur in the mill process is shorter, wherein excess sulfur is recovered from the liquid circulation rather than from later stages of the process, such as gas or fly ash formed in the mill process. Another advantage is that the adjustment of the S/Na balance of the pulp mill can be achieved in a simpler and faster way. Moreover, sulfur can be recovered in its elemental form without simultaneous loss of sodium. This reduces the need to add make-up NaOH to adjust the sulfidation of the mill, thereby reducing costs and avoiding unnecessary use of chemicals. Thus, the S/Na balance of the pulp mill can be adjusted in a cost-effective and environmentally friendly manner.
Recovery of spent cooking chemicals in a pulp mill is referred to as a pulp mill's liquid cycle or chemical recovery cycle. The spent cooking chemicals may be burned in a recovery boiler, forming a molten "melt" which may be dissolved in a liquid. The liquid so formed may be referred to as green liquid due to the characteristic green color. The green liquor may be used to prepare white liquor for the pulping process. The liquid circulation is designed to recover chemicals used in pulping.
Sulfur balance control is important in pulp mills. Along withSulfur is treated with typical sodium sulfide (Na 2 S) form is introduced into the cooking process and sulfur must also be removed from the chemical recovery cycle in some form to avoid excessive sulfur content in the cycle. Excessive sulfur levels and unnecessarily low sulfur levels in the chemical recovery cycle may cause operational problems resulting in, for example, poor slurry quality, increased plant energy consumption, and decreased plant capacity. The S/Na balance of the mill is related to the degree of sulfidation. The sulfidation degree is Na in white liquor of pulp mill 2 Percentage value of the ratio between S and active base. The active alkali refers to NaOH and Na 2 S, S. The optimum degree of vulcanization depends on several factors, such as wood species, alkali usage, cooking temperature and desired properties in the final product. Typically, the degree of vulcanization may vary between 20 and 50%.
Contains Na 2 The green liquor of S and NaHS is an important part of the liquid circulation, which is responsible for recovery of chemicals used in pulping. As disclosed above, the white liquor formed from the green liquor also contains sulfides. Thus, the green liquor stream diverted from the recovery boiler or the green or white liquor stream diverted from the process thereafter represents a convenient source of material for adjusting the S/Na balance of the pulp mill by removing sulfur from the chemical recovery cycle.
According to one aspect of the invention, at least a portion of a sulfide-containing pulp mill liquid stream, such as a green or white liquor stream, is transferred to a bioreactor. The sulfide-containing liquid may then be subjected to biological oxidation in a bioreactor by means of sulfur oxidizing microorganisms to form elemental sulfur. Elemental sulfur may then be recovered.
According to another aspect of the invention, at least a portion of a sulfide-containing pulp mill liquid stream, such as a green or white liquor stream, may be transferred to a stripper. The sulphidic mill liquor may be stripped with an acidic reagent in a stripper. The acidic reagent lowers the pH of the pulp mill liquor. In this way, sulphides in the mill liquor can be converted into gaseous H 2 S, S. Thus, H-containing can be obtained 2 S gas stream and residual pulp mill liquid stream. Then H is contained 2 The gas stream of S is washed in a scrubber with an aqueous scrubbing solution containing an alkaline reagent, such as NaOH. When in contact with,H 2 S reacts with an alkaline reagent to produce a sulfide-containing (e.g., na 2 S and NaHS) that transfer themselves from the gas phase to the liquid phase during the reaction, selective sulfide conversion can be achieved. The aqueous spent wash solution containing sulfides is then subjected to biological oxidation in a bioreactor by means of sulfur oxidizing microorganisms to form elemental sulfur. Elemental sulfur may then be recovered.
Accordingly, there is provided a method for adjusting the S/Na balance of a pulp mill, the method comprising:
transferring the sulphide containing aqueous mill liquor to a bioreactor,
biologically oxidizing the sulphide-containing aqueous mill liquor in an oxidation reaction by means of sulphur oxidising microorganisms in a bioreactor, thereby producing an aqueous suspension containing elemental sulphur, and
separating the elemental sulphur from the aqueous suspension in a sulphur separation unit located downstream of the bioreactor, thereby obtaining a residual solution and a precipitate containing elemental sulphur.
Optionally, the method for adjusting the S/Na balance of a pulp mill may comprise:
transferring the aqueous mill liquor containing sulphides to a stripper,
stripping the aqueous mill liquor containing sulphides with an acidic reagent in a stripper, thereby obtaining a sulphidic aqueous slurry containing H 2 S gas stream and residual pulp mill liquid stream,
washing the aqueous scrubbing solution containing the alkaline reagent with a scrubber located downstream of the stripper 2 S, thereby causing at least some H 2 S reacts with the alkaline reagent to produce a residual gas stream and an aqueous spent wash solution containing sulfides,
introducing an aqueous spent wash solution into a bioreactor,
biologically oxidizing an aqueous spent washing solution containing sulphide in an oxidation reaction by means of sulphur oxidising microorganisms in a bioreactor, thereby producing an aqueous suspension containing elemental sulphur, and
separating the elemental sulphur from the aqueous suspension in a sulphur separation unit located downstream of the bioreactor, thereby obtaining a residual solution and a precipitate containing elemental sulphur.
The object according to the invention is further described in the appended claims.
Brief description of the drawings
Figure 1 shows by way of example a flow chart of a system configured to adjust the S/Na balance of a pulp mill,
Figure 2a shows by way of example a variant of a flow chart of a system configured to adjust the S/Na balance of a pulp mill,
figure 2b shows by way of example another variant of a flow chart of a system configured to adjust the S/Na balance of a pulp mill,
figure 3 shows by way of example a stripper configured to separate sulfur from a pulp mill liquid stream,
FIG. 4 shows, by way of example, a scrubber configured to separate sulfur from a pulp mill liquid stream, an
Fig. 5 shows, by way of example, a bioreactor configured to separate sulfur from a pulp mill liquid stream.
These figures are schematic. The numbers are not to any particular scale.
Detailed Description
The term "scrubber" refers to an air pollution control device that is used to remove particulates or compounds from the exhaust gas stream of a pulp mill. The aqueous solution may be introduced into a scrubber to collect unwanted contaminants from the gas stream into the aqueous spent scrubbing solution.
The term "efficiency" refers to the quantitative ratio of yield to total input. Unless otherwise indicated, efficiency in this context is calculated as a percentage of the theoretical maximum that a given total input can produce. In other words, efficiency is expressed as a percentage of what would be expected in an ideal situation.
The term "weak malodorous gas" generally refers to sulfur concentrations less than 0.5g/m 3 Is a gas of (a) a gas of (b). The weak malodorous gas may also be referred to as diluted malodorous gas. In a pulp mill environment, weak malodorous gases may come from, for example, wood chip pre-steaming, screening, pulp washing, melt dissolving tanks (smelt dissolver) and venting of various tanks (ventilation ofvarious tanks).
The term "strong malodorous gas" generally means a sulfur concentration of greater than 5g/m 3 Is a gas of (a) a gas of (b). In a pulp mill environment, strong malodorous gases may come from, for example, digesters, evaporation equipment, and condensate strippers.
The term "volumetric flow rate" refers to the volume of fluid passing per unit time.
The term "mass flow" refers to the mass of material passing per unit time.
In the context of the present specification, the term "sulphide" is meant to include HS-or S 2- A partial compound or substance. These compounds or substances include, for example, naHS and Na 2 S and their hydrates.
The term "clarification" refers to a process of making a fluid (typically a liquid) clear by removing impurities or solid matter.
The term "aerated" refers to the supply of oxygen or air. Aeration is the process of flowing air through, mixing with, or dissolving in a liquid, thereby transferring oxygen into the liquid (e.g., an aqueous solution).
In chemical pulp production, digestion is used to recover fibers from wood chips in digesters by using chemicals and heat to remove lignin bound to the fibers and to remove wood extractives, which may later cause foaming and precipitation in the process. Thus, chemicals are often used in pulping processes that dissolve as much lignin and as little cellulose as possible. Generally, the process for making bleached chemical pulps includes pulping, washing, screening, bleaching and cleaning stages. Sulfate cooking (also known as Kraft cooking (Kraft cooking) or pulping) is the most common pulp production method today, which uses sodium hydroxide (NaOH) and sodium sulfide (Na) 2 S) a mixture of S). The cooking process may be based on batch cooking or continuous cooking comprising one digester or several digesters. The chemicals required for the process are mixedUsed in the form of a mixture called white liquor.
During the pulping process, white liquor sodium sulfide (Na 2 S) and sodium hydroxide (NaOH) react with water to form sulfhydryl groups (HS) according to formulas 1 and 2 - ) And hydroxyl (OH-).
Na 2 S+H 2 O→2Na + +HS- +OH- (1)
The result of the pulping process is the formation of black liquor. Pulp from the digester contains fibers and spent cooking liquor (black liquor). A large amount of chemicals are used in chemical pulp production and recovery and reuse of these chemicals is required. The main process units in the chemical recovery system of a pulp mill are evaporation of black liquor, combustion of the evaporated liquor in a recovery boiler and caustic-ing (including the production of quicklime).
Recovery boilers are used to recover cooking chemicals. Upon combustion, the cooking chemicals form a molten "melt" at the bottom of the recovery boiler. The melt may be dissolved in a liquid. The liquid so formed may be referred to as green liquid due to the characteristic green color. The green liquor may be used to prepare white liquor for the pulping process. The circulation of these spent cooking chemicals is called liquid circulation. The liquid circulation is designed to recover chemicals used in pulping. In particular, recovery boilers are intended for the recovery of sodium carbonate (Na 2 CO 3 ) And sodium sulfide (Na) 2 S). The green liquor is clarified and caustic treated with quicklime, during which process Na is added 2 CO 3 Converted into NaOH. Removing NaOH and Na 2 In addition to S, the white liquor also contains other sodium salts, such as sodium sulfate (Na 2 SO 4 ) And small amounts of sulfites and chlorides.
Sulfur balance control is important in pulp mills. As sulfur is introduced into the cooking process, it must also be removed from the chemical recovery cycle to avoid excessive sulfur content in the cycle. The S/Na balance of the mill is related to the degree of sulfidation. The sulfidation degree is Na in white liquor of pulp mill 2 Ratio between S and active basePercent value. The active alkali refers to NaOH and Na 2 S, S. The degree of vulcanization may generally vary between 20 and 50%. Equation 3 may be used to represent the degree of vulcanization. Na (Na) 2 The amounts of S and NaOH can be expressed in grams of NaOH equivalents or as a percentage of dry wood. Standard NaOH SCAN-N30:85 and Na may be used 2 S SCAN-N31:94 determines the degree of sulfidation of the mill. By adding make-up NaOH to the chemical recovery cycle, the sulfidation of the pulp mill can be maintained at a desired level. However, this results in additional costs and requires the use of unnecessary chemicals.
A method and system for adjusting S/Na balance of a pulp mill by removing sulfur compounds from a chemical recovery cycle of the pulp mill and for processing the sulfur compounds into elemental sulfur of high intrinsic value is disclosed. Chemically, sulfur reacts with almost all elements, except some rare metals and rare gases. Elemental sulfur may be used as a precursor for other chemicals, such as sulfuric acid. In addition, the disclosed methods and systems enable sulfur recovery without loss of sodium. The recovery of sulfur without recovery of sodium can be used to adjust the S/Na balance of the pulp mill.
Fig. 1 shows, by way of example, a system 100 for adjusting the S/Na-balance of a kraft pulp mill. The system 100 includes a bioreactor 102 and a sulfur separation unit 106 downstream of the bioreactor 102.
In a method that may be implemented by the system 100, aqueous mill liquor 109 containing sulfide is collected. The pH of aqueous mill liquor 109 is alkaline. The pH of the sulfide-containing aqueous slurry 109 may be about 14. The aqueous mill liquor 109 may comprise, for example, a mill green liquor stream or a mill white liquor stream.
The mill green liquor stream may come from a recovery boiler where the concentrated black liquor is burned. Combustion forms a molten "melt" at the bottom of the recovery boiler. The melt containing, for example, na 2 CO 3 And Na (Na) 2 S, S. The melt may be dissolved in a liquidMay be, for example, water or a thin white liquor. The liquid thus formed is called green liquid due to the characteristic green color. The green liquor contains sulfides, e.g. Na 2 S and NaHS. The mill green stream may be clarified in a clarifier unit to provide aqueous mill liquor 109, or the mill green stream may be used directly in the process of the invention. In the latter case, the mill green stream corresponds to the aqueous mill liquor 109.
Aqueous slurry plant liquid 109 is transferred to bioreactor 102. Fig. 5 shows, by way of example, the bioreactor 102, 202 with reference to fig. 1, 2a and 2 b. The temperature of aqueous slurry plant liquid 109 is above room temperature prior to entering bioreactor 102. Preferably, the temperature of aqueous slurry plant liquid 109 is in the range of 40 to 60 ℃ prior to entering bioreactor 102. If desired, the temperature of aqueous slurry 109 may be reduced by a heat exchanger disposed upstream of bioreactor 102. In bioreactor 102, aqueous slurry plant liquid 109 containing sulfide is biologically oxidized in an oxidation reaction. The oxidation is carried out by means of sulfur oxidizing microorganisms. In an exemplary pulp mill producing one million tons of air dried pulp per year, the volumetric flow rate of aqueous pulp mill liquid 109 transferred to bioreactor 102 may be 6.9 m/hr 3 . Na of aqueous slurry plant liquid 109 transferred to bioreactor 102 2 The S concentration may be 46.8g/l.
The sulfur oxidizing microorganisms may be autotrophic, heterotrophic, or facultative aerobic bacteria. The sulfur oxidizing microorganism may be alkalophilic. The sulfur oxidizing microorganisms may include, for example, bacteria of the genus Thiobacillus (Thiobacillus) and sulfur microcosmic (thiomicrocopora). Bacteria capable of oxidizing sulfides to elemental sulfur can be obtained from, for example, geothermal spas, ocean geothermal spouts, sulfide cave systems, sulfide-enriched industrial sites, sewage sludge, soil, salt-biogas, alkaline lakes, and cold springs. Alkalophilic sulfur oxidizing bacteria such as Alkallimicrozyme (Thioalklimichibium), vibrio thioalkali (Thioalklivibbrio) and Alkallispira thioalkali (Thioalklispira) can be isolated from alkaline lakes. They may have halophilic or salt tolerance to varying degrees. The sulfur oxidizing microorganism may have at least one of the following properties: the optimum pH is above 9, generally below 10.5, in particularIs about 9.5; capable of oxidizing at least H 2 S/HS-; growing in a temperature range of 10-65 ℃; tolerance to NaCl and sodium carbonate.
Bioreactor 102 may be inflated with a gas 105 from a pulp mill that contains air and/or a weak malodorous gas. In the oxidation reaction, most of the sulfides of aqueous mill liquor 109 are oxidized to elemental sulfur. The efficiency of the oxidation reaction may be equal to or greater than 95%. Since the chemical stability of the elemental sulphur produced decreases with increasing pH and temperature, the temperature inside the bioreactor should not exceed 65 ℃. The pH of the reaction medium inside bioreactor 102 may be between 8 and 11. By aerating the bioreactor 102 with a weak malodorous gas, the pH of the reaction medium can be lowered. Bioreactor 102 may be a hybrid reactor. The system 100 may contain more than one bioreactor. The bioreactors may be arranged in parallel.
The oxidation reaction produces an aqueous suspension 103 containing elemental sulphur. The oxidation reaction also produces a gas stream 104. The gas stream 104 may proceed from the bioreactor 102 to the treatment of weak malodorous gases of the pulp mill. The treatment of the weak malodorous gas may be performed in the recovery boiler in such a way that the weak malodorous gas is fed into the combustion air of the recovery boiler.
The aqueous suspension 103 containing elemental sulphur from the bioreactor 102 is led to a sulphur separation unit 106. In a sulfur separation unit 106, elemental sulfur is separated from the aqueous suspension 103. Thereby obtaining a residual solution 108 and a precipitate 107 containing elemental sulphur. The sulfur separation unit 106 may be a conical separator. The separation may be performed by, for example, filtration, sedimentation or flocculation. In an exemplary pulp mill producing one million tons of air dried pulp per year, the amount of elemental sulfur produced may be 128kg per hour. The residual solution 108 from the sulfur separation unit 106 from which the precipitate 107 has been separated may be directed to causticization.
Fig. 2a and 2b show by way of example another system for separating sulfur from a pulp mill liquid stream. The system 200 includes a stripper 210, a scrubber 214 downstream of the stripper 210, a bioreactor 202 downstream of the scrubber 214, and a sulfur separation unit 206 downstream of the bioreactor 202.
In a method that may be implemented by system 200, aqueous mill liquor 109 containing sulfide is collected. The pH of aqueous mill liquor 109 is alkaline. The pH of the sulfide-containing aqueous slurry 109 may be about 14. The aqueous mill liquor 109 may comprise, for example, a mill green liquor stream or a mill white liquor stream. Aqueous mill liquor 109 is transferred to stripper 210. In an exemplary pulp mill producing one million tons of air dried pulp per year, the volumetric flow rate of aqueous pulp mill liquid 109 transferred to stripper 210 may be 54.2 m/hour 3 . Na of aqueous slurry plant liquid 109 transferred to stripper 210 2 The S concentration may be 46.8g/l.
Aqueous mill liquor 109 containing sulfides is stripped with an acidic reagent in stripper 210. The acidic reagent may be, for example, carbon dioxide (CO 2 ) Or an acidic solution. A stripping fluid stream 213 comprising an acidic reagent is fed into stripper 210. The stripping fluid stream 213 may comprise, for example, pure carbon dioxide or flue gas. In stripper 210, stripping fluid stream 213 lowers the pH of aqueous mill liquor 109, thereby causing H to form from the sulfides of aqueous mill liquor 109 2 S, S. At the time of stripping, the pH of aqueous slurry plant liquid 109 may be 7 or less.
As shown in fig. 3, the stripping in stripper 210 is performed in countercurrent. Aqueous mill liquor 109 containing sulphide is fed into stripper 210 at an upper portion of stripper 210 and is arranged to flow down to a lower portion of stripper 210. The stripping fluid stream 213 is fed into the stripper 210 at a lower portion of the stripper 210 and is arranged to flow upward toward an upper portion of the stripper 210. Stripper 210 may be a tray column or a packed bed column.
Stripping to produce a product containing H 2 S gas stream 211 and residual mill liquid stream 212. H of the air flow 211 2 The S concentration may be 99% by volume. The residual pulp mill liquid stream 212 may be fed back into the chemical recovery cycle of the pulp mill. In an exemplary pulp mill producing one million tons of air dried pulp per year, H is contained 2 The mass flow rate of the gas stream 211 of S may be 553kg per hour. The volumetric flow rate of the residual pulp flow 212 may be 54.2 m/hour 3 . Residual pulp mill liquorNa in the body stream 212 2 The S concentration may be 23.4g/l.
Fig. 4 shows, by way of example, with reference to fig. 2a and 2b, a scrubber 214. Will contain H 2 The gas stream 211 of S is fed to scrubber 214. In scrubber 214, the aqueous scrubbing solution 215 is used to scrub the aqueous scrubbing solution containing H 2 S air flow 211. The pH of the wash solution 215 may be adjusted with an alkaline reagent. The alkaline agent-containing stream 216 can be configured to feed the alkaline agent into the aqueous wash solution 215. The alkaline reagent may be, for example, a NaOH solution or an oxidized white liquor. The pH of the aqueous wash solution 215 may be above 8. Preferably, the pH of the aqueous wash solution 215 is above 11.5. The pH of the aqueous wash solution 215 may be in the range of 12 to 14. Higher pH can increase the efficiency of the wash. When NaOH is used as the alkaline agent, in an exemplary pulp mill producing one million tons of air dried pulp per year, the mass flow rate of NaOH fed into the aqueous wash solution 215 may be 25kg per hour.
In the scrubber 214, the H-containing can be realized 2 The gas stream 211 of S is in sufficient contact with the aqueous scrubbing solution 215. At least some H in the gas stream 211 2 S reacts with alkaline agents in the aqueous wash solution 215 to form sulfides, e.g., na 2 S and NaHS. A residual gas stream 217 and an aqueous spent scrubbing solution 201 containing sulfides are produced in scrubber 214. Na of aqueous spent wash solution 201 2 The S/NaHS mixture ratio depends on the pH of the aqueous spent wash solution 201. The residual gas stream 217 may proceed from scrubber 214 to the treatment of strong malodorous gases of the pulp mill. The treatment of the strong malodorous gas may comprise, for example, burning the gas in a recovery boiler.
Scrubber 214 may be a packed bed column type absorber. Scrubber 214 provides a direct contact area between the gas and the liquid. Advantageously, the system 100, 200 may comprise at least one conduit configured to direct the residual gas stream 217 from the scrubber 214 into the pulp mill recovery boiler. This allows at least some of the residual gas stream 217 from scrubber 214 to be directed into a pulp mill recovery boiler, thereby enabling a chemical recovery cycle that recirculates chemicals from residual gas stream 217 to the pulp mill Is a kind of medium. Thus, by removing the chemical from containing H 2 The introduction of the S gas stream 211 back into the chemical recovery cycle of the mill may further enhance the method and system that can achieve adjustment of the S/Na balance of the mill by separating sulfur compounds from the sulfide-containing mill liquor and oxidizing the sulfide to elemental sulfur with microorganisms.
The aqueous spent wash solution 201, 201a containing sulfides is introduced into the bioreactor 202 (fig. 5). The temperature of the aqueous spent wash solution 201, 201a prior to entering the bioreactor 202 is above room temperature. Preferably, the temperature of the aqueous spent wash solution 201, 201a prior to entering the bioreactor 202 is in the range of 40 to 60 ℃. In the bioreactor 202, the aqueous spent wash solution 201, 201a containing sulfides is biologically oxidized in an oxidation reaction. The oxidation is carried out by means of sulfur oxidizing microorganisms.
According to the embodiment shown in fig. 2b, at least some of the aqueous spent wash solution 201b is recycled back to the scrubber 214 with a pump 218. Thus, the aqueous spent wash solution 201 is split into two portions 201a and 201b. With this arrangement, sulfur compounds in the gas stream 211 can be more efficiently converted to sulfur compounds.
Bioreactor 202 may be inflated with a gas 205 from a pulp mill that contains air and/or a weak malodorous gas. In the oxidation reaction, most of the sulphide in the aqueous spent wash solution 201, 201a is oxidized to elemental sulphur. The efficiency of the oxidation reaction may be equal to or greater than 95%. Since the chemical stability of the elemental sulphur produced decreases with increasing pH and temperature, the temperature inside the bioreactor should not exceed 65 ℃. The pH of the reaction medium inside bioreactor 202 may be between 8 and 11. By aerating the bioreactor 202 with a weak malodorous gas, the pH of the reaction medium can be lowered. In this way, by aerating bioreactor 202 with a weak malodorous gas that can lower the pH of the reaction medium, it is possible to compensate for using a slightly higher pH in scrubber 214 than the optimal pH for bioreactor 202. Bioreactor 202 may be a hybrid reactor. The system 200 may include more than one bioreactor. The bioreactors may be arranged in parallel.
The oxidation reaction produces an aqueous suspension 203 containing elemental sulfur. The oxidation reaction also produces a gas stream 204. The gas stream 204 may proceed from the bioreactor 202 to the treatment of weak malodorous gases of the pulp mill. The treatment of the weak malodorous gas may be performed in the recovery boiler in such a way that the weak malodorous gas is fed into the combustion air of the recovery boiler. Advantageously, the system 100, 200 may comprise at least one conduit configured to direct the gas stream 104, 204 from the bioreactor 102, 202 into a pulp mill recovery boiler. This allows at least some of the gas streams 104, 204 from the bioreactors 102, 202 to be directed into the pulp mill recovery boiler, enabling recirculation of chemicals from the gas streams 104, 204 into the chemical recovery cycle of the pulp mill. Thus, by introducing chemicals from the gas streams 104, 204 back into the chemical recovery cycle of the mill, a method and system capable of achieving regulation of the S/Na balance of the mill by separating sulfur compounds from the mill liquor containing the sulfides and oxidizing the sulfides to elemental sulfur with microorganisms can be further enhanced.
The aqueous suspension 203 containing elemental sulphur from the bioreactor is led to a sulphur separation unit 206. In a sulfur separation unit 206, elemental sulfur is separated from the aqueous suspension 203. Thereby obtaining residual solutions 208a, 208b and a precipitate 207 containing elemental sulphur. The sulfur separation unit 206 may be a conical separator. The separation may be performed by, for example, filtration, sedimentation or flocculation. In an exemplary pulp mill producing one million tons of air dried pulp per year, the amount of elemental sulfur produced may be 500kg per hour. The mass flow rate of the residual solutions 208a, 208b relative to sulfur may be 10kg per hour.
In the embodiment shown in fig. 2b, at least some of the aqueous spent wash solution 201b is recycled back to the scrubber 214 by pump 218, which embodiment enables the use of a smaller sulfur separation unit 206 than the system disclosed in fig. 2 a. Since the sulfur compounds in the gas stream 211 are more efficiently converted to sulfides, the volume of the aqueous suspension 203 containing elemental sulfur can be smaller, thus requiring a smaller unit to separate the residual solution 208 and the precipitate 207 containing elemental sulfur.
At least some of the residual solution 208a from the sulfur separation unit 206 from which the precipitate 207 has been separated may be directed back into the scrubber 214 to replenish the aqueous scrubbing solution 215. Thus, sulfur compounds in the residual solution 208a that may not be oxidized may be directed back to the bioreactor 202 for oxidation. In addition, recycling the liquid reduces the need for fresh water and reduces unnecessary use of valuable natural resources. The remaining solution 208b may be fed into the chemical recovery cycle of the pulp mill.
Many variations of the method and system will be apparent to those skilled in the art in light of the above description. Such obvious variations are within the full intended scope of the appended claims.

Claims (56)

1. A method for adjusting S/Na balance of a pulp mill, the method comprising:
transferring the sulphide containing aqueous mill liquor (109) to a bioreactor (102),
-inflating the bioreactor (102) with a gas (105) from the pulp mill comprising a weak malodorous gas, said weak malodorous gas being having a sulfur concentration of less than 0.5g/m 3 Is a gas of (a) and (b),
-subjecting the sulphide-containing aqueous mill liquor (109) to biological oxidation in an oxidation reaction by means of sulphur oxidising microorganisms in a bioreactor (102), thereby producing an aqueous suspension (103) containing elemental sulphur, and
separating elemental sulphur from the aqueous suspension (103) in a sulphur separation unit (106) located downstream of the bioreactor (102), thereby obtaining a residual solution (108) and a precipitate (107) containing elemental sulphur,
wherein the aqueous mill liquor (109) is green liquor or white liquor.
2. The method as recited in claim 1, further comprising:
-clarifying the mill liquid stream in a clarifier unit, thereby providing an aqueous mill liquid (109).
3. The method according to claim 1 or 2, wherein
The aqueous mill liquor (109) has a temperature above room temperature prior to entering the bioreactor (102).
4. The method of claim 1 or 2, further comprising:
-directing at least some of the gas stream (104) from the bioreactor (102) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (104) into a chemical recovery cycle of the pulp mill.
5. A method according to claim 3, further comprising:
-directing at least some of the gas stream (104) from the bioreactor (102) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (104) into a chemical recovery cycle of the pulp mill.
6. A method for adjusting S/Na balance of a pulp mill, the method comprising:
transferring the aqueous mill liquor (109) containing sulphide to a stripper (210),
-stripping the aqueous mill liquor (109) containing sulphides with an acidic reagent in a stripper (210) to obtain a slurry containing H 2 S gas stream (211) and residual pulp mill liquid stream (212), the acidic reagent being carbon dioxide,
-washing the aqueous washing solution (215) containing alkaline agent with a washing liquid containing H in a scrubber (214) downstream of the stripper (210) 2 S gas flow (211) such that at least some H 2 S reacts with an alkaline reagent to produce a residual gas stream (217) and an aqueous spent wash solution (201, 201 a) containing sulfides,
-inflating the bioreactor (202) with a gas (205) from the pulp mill comprising a weak malodorous gas, said weak malodorous gas being having a sulfur concentration of less than 0.5g/m 3 Is a gas of (a) and (b),
-subjecting an aqueous spent washing solution (201, 201 a) containing sulphide to biological oxidation in an oxidation reaction by means of sulphur oxidising microorganisms in a bioreactor (202), thereby producing an aqueous suspension (203) containing elemental sulphur, and
separating elemental sulphur from the aqueous suspension (203) in a sulphur separation unit (206) located downstream of the bioreactor (202), thereby obtaining a residual solution (208 a,208 b) and a precipitate (207) containing elemental sulphur,
wherein the aqueous mill liquor (109) is green liquor or white liquor.
7. The method as recited in claim 6, further comprising:
-directing at least some of the residual solution (208 a) from which the precipitate (207) has been separated back into the scrubber (214) to replenish the aqueous scrubbing solution (215).
8. The method of claim 6 or 7, further comprising:
-directing at least some of the aqueous spent wash solution (201 b) back into the scrubber (214) for re-scrubbing by a pump (218).
9. The method of claim 6 or 7, further comprising:
-clarifying the mill liquid stream in a clarifier unit, thereby providing an aqueous mill liquid (109).
10. The method as recited in claim 8, further comprising:
-clarifying the mill liquid stream in a clarifier unit, thereby providing an aqueous mill liquid (109).
11. The method according to claim 6 or 7, wherein
-aqueous mill liquor (109) or
-aqueous spent washing solution (201, 201 a)
Has a temperature above room temperature prior to entering the bioreactor (202).
12. The method of claim 8, wherein
-aqueous mill liquor (109) or
-aqueous spent washing solution (201, 201 a)
Has a temperature above room temperature prior to entering the bioreactor (202).
13. The method of claim 9, wherein
-aqueous mill liquor (109) or
-aqueous spent washing solution (201, 201 a)
Has a temperature above room temperature prior to entering the bioreactor (202).
14. The method of claim 10, wherein
-aqueous mill liquor (109) or
-aqueous spent washing solution (201, 201 a)
Has a temperature above room temperature prior to entering the bioreactor (202).
15. The method of claim 6 or 7, further comprising:
-adjusting the pH of the aqueous washing solution (215) with an alkaline agent such that the pH of the aqueous washing solution (215) is higher than 8.
16. The method as recited in claim 8, further comprising:
-adjusting the pH of the aqueous washing solution (215) with an alkaline agent such that the pH of the aqueous washing solution (215) is higher than 8.
17. The method as recited in claim 9, further comprising:
-adjusting the pH of the aqueous washing solution (215) with an alkaline agent such that the pH of the aqueous washing solution (215) is higher than 8.
18. The method as recited in claim 10, further comprising:
-adjusting the pH of the aqueous washing solution (215) with an alkaline agent such that the pH of the aqueous washing solution (215) is higher than 8.
19. The method as recited in claim 11, further comprising:
-adjusting the pH of the aqueous washing solution (215) with an alkaline agent such that the pH of the aqueous washing solution (215) is higher than 8.
20. The method of any one of claims 12-14, further comprising:
-adjusting the pH of the aqueous washing solution (215) with an alkaline agent such that the pH of the aqueous washing solution (215) is higher than 8.
21. The method of claim 6 or 7, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
22. The method as recited in claim 8, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
23. The method as recited in claim 9, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
24. The method as recited in claim 10, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
25. The method as recited in claim 11, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
26. The method of any one of claims 12-14, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
27. The method as recited in claim 15, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
28. The method of any one of claims 16-19, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
29. The method as recited in claim 20, further comprising:
-directing at least some of the residual gas stream (217) from the scrubber (214) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the residual gas stream (217) into a chemical recovery cycle of the pulp mill.
30. The method of claim 6 or 7, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
31. The method as recited in claim 8, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
32. The method as recited in claim 9, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
33. The method as recited in claim 10, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
34. The method as recited in claim 11, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
35. The method of any one of claims 12-14, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
36. The method as recited in claim 15, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
37. The method of any one of claims 16-19, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
38. The method as recited in claim 20, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
39. The method as recited in claim 21, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
40. The method of any one of claims 22-25, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
41. The method as recited in claim 26, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
42. The method as recited in claim 27, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
43. The method as recited in claim 28, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
44. The method of claim 29, further comprising:
-directing at least some of the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
45. A system (100) arranged to adjust S/Na balance of a pulp mill, the system (100) comprising:
means configured to collect a sulfide-containing aqueous mill liquor (109), said aqueous mill liquor (109) being green liquor or white liquor,
-one or more conduits configured to direct aqueous pulp mill liquid (109) into a bioreactor (102), wherein the bioreactor (102) is configured to be aerated with a gas (105) from the pulp mill comprising a weak malodorous gas, the weak malodorous gas being having a sulfur concentration of less than 0.5g/m 3 Is a gas of (a) and (b),
-a bioreactor (102) configured to oxidize an aqueous pulp mill liquid (109) with sulfur oxidizing microorganisms, whereby the bioreactor (102) is configured to produce an aqueous suspension (103) containing elemental sulfur, and
-a sulphur separation unit (106) located downstream of the bioreactor (102), the sulphur separation unit (106) being configured to produce a residual solution (108) and a precipitate (107) containing elemental sulphur.
46. The system (100) of claim 45, further comprising at least one conduit configured to direct the gas stream (104) from the bioreactor (102) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (104) into a chemical recovery cycle of the pulp mill.
47. The system (100) according to claim 45 or 46, the system (100) comprising more than one bioreactor (102).
48. A system (200) arranged to adjust S/Na balance of a pulp mill, the system (200) comprising:
means configured to collect a sulfide-containing aqueous mill liquor (109), said aqueous mill liquor (109) being green liquor or white liquor,
one or more conduits configured to direct aqueous mill liquor (109) into a stripper (210),
-a stripper (210) configured to strip the aqueous mill liquor (109) with an acidic reagent, whereby the stripper (210) is configured to produce a slurry containing H 2 S gas stream (211) and residual pulp mill liquid stream (212), the acidic reagent being carbon dioxide,
-a scrubber (214) downstream of the stripper (210), the scrubber (214) being configured to scrub the aqueous scrubbing solution (215) containing the alkaline reagent with a gas containing H 2 S, whereby the scrubber (214) is configured to produce a residual gas stream (217) and an aqueous spent scrubbing solution (201, 201 a) containing sulfides,
One or more conduits configured to direct an aqueous spent wash solution (201, 201 a) containing sulfide into a bioreactor (202),
-a bioreactor (202) downstream of the scrubber (214), the bioreactor (202) being configured to oxidize the content with sulfur oxidizing microorganismsAn aqueous spent wash solution (201, 201 a) having sulphide whereby the bioreactor (202) is configured to produce an aqueous suspension (203) containing elemental sulphur, wherein the bioreactor (202) is configured to be aerated with a gas (205) from a pulp mill comprising a weak malodorous gas, the weak malodorous gas being having a sulphur concentration of less than 0.5g/m 3 And (2) gas of
-a sulphur separation unit (206) located downstream of the bioreactor (202), the sulphur separation unit (206) being configured to produce a residual solution (208 a,208 b) and a precipitate (207) containing elemental sulphur.
49. The system (200) of claim 48, the system (200) further comprising a pump (218) and conduit configured to direct at least some of the aqueous spent wash solution (201 b) back into the scrubber (214) for re-scrubbing.
50. The system (200) of claim 48 or 49, further comprising at least one gas stream (217) configured to direct residual gas from the scrubber (214) into a pulp mill recovery boiler to effect chemical removal from the H-containing stream 2 The residual gas stream (211) of S is recycled to the conduit in the chemical recovery cycle of the pulp mill.
51. The system (200) of claim 48 or 49, further comprising at least one conduit configured to direct a gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler, thereby enabling recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
52. The system (200) of claim 50, further comprising at least one conduit configured to direct the gas stream (204) from the bioreactor (202) into a pulp mill recovery boiler to effect recirculation of chemicals from the gas stream (204) into a chemical recovery cycle of the pulp mill.
53. The system (200) according to claim 48 or 49, the system (200) comprising more than one bioreactor (202).
54. The system (200) of claim 50, the system (200) comprising more than one bioreactor (202).
55. The system (200) of claim 51, the system (200) comprising more than one bioreactor (202).
56. The system (200) of claim 52, the system (200) comprising more than one bioreactor (202).
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